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Overlooked Diseases: Part I

Rhizoctonia, Powdery Mildew and Early Blight

Published in the April 2009 IssuePublished online: Apr 30, 2009

Like maladies and threats in other facets of the world, it seems that only a select few crop pests get their names hoisted up in agricultural marquee lights. Growers are all too aware of certain diseases, such as late blight. But what about the scores of other microscopic monsters that get far less attention?

Based on five years of plot work at the Aberdeen Research Station in southern Idaho, soil and disease management specialists have presented an overview of their studies focusing on a handful of lesser known pathogens. Presented by Phil Wharton, Jeff Miller and Phil Nolte at the 2009 Potato Conference in Pocatello, Idaho, the roster includes Rhizoctonia, black scurf, powdery mildew, early blight and brown leaf spot, gray mold, black dot and white mold. (This issue will focus on the first half; our May issue will address the latter.)

Just because some diseases are lesser known doesn't mean they are any less detrimental to the health of the crop. In fact, their often covert nature is a big part of their potency.

"Take a disease like early blight or brown spot," said Nolte. "Here you have a disease that has the ability to infect the plants, and not actually show a symptom of its presence for a week, maybe longer. There are a number of cases where individuals have thought they were applying a chemical on a timely basis, when in fact there was already a lot of infection taking place."

While the data are hardly any substitute for large-scale applications in the real world, confidence is high regarding the diseases' behavior, vectors and measured effects from various control methods.

Rhizoctonia

This particular disease is not often on the minds of growers, but it certainly causes its fair share of problems.

According to Nolte, you can identify Rhizoctonia by its most conspicuous sign-black scurf. During in-field inspection also look for stem girdling and collapse, stolon girdling and aborted tubers.

Especially in potatoes, he says, it's the younger tissues that are most susceptible to this disease. All it takes is the perfect cocktail of conditions to get it started.

"What often happens is that you'll get a blast of [colder] temperature coming in, so you'll have the plant in a susceptible state, and you'll have the fungus in a state where it's ready to go," he said. The two factors clash together and there you have it. It goes a long way in explaining why Rhizoctonia can be such a major problem some years and a very minor one other years.

Black scurf is easy to spot if you know what to look for. At this stage the fungus forms dark, hard masses on the tuber. Early in the season, it attacks pre-emergent sprouts underground before they emerge from the soil. The results: delayed emergence, poor and uneven stands and weakened plants

Powdery Mildew

At the University of Idaho conference, Nolte also presented on powdery mildew, another often overlooked pathogen that affects crops to varying degrees.

"You can see this organism oftentimes on the weeds in your lawn," he said. "Most of the diseases that we deal with are actually imbedded in the tissues in the plant, but powdery mildews are almost completely on the outside. Thankfully, that makes it somewhat easier to manage."

Nolte notes another "oddball thing" is that unlike most other diseases, powdery mildew requires no droplets of water, or what researchers term as "free moisture."

Still, this mostly superficial fungus is likely to pop up in areas where a grower may still be using flood irrigation or other areas prone to high humidity.

Some specific product recommendations from the conference include:

JMS Stlet-Oil at 3 to 6 qt/100 gal. water.

Kumulus DF at 3 to 5 lb/A at 10- to 14-day intervals.

Microthiol Disperss at 5 lb/A.

Sulfur 6 Flowable 6 at 3 to 4 pt/A.

Thiolux at 3 to 10 lb/A every 10 to 14 days.

Quadris at 6 to 15.5 fl oz/A.

Quadris Opti at 1.6 pints/A on a 7- to 14-day interval.

Early Blight and Brown Leaf Spot

One reason that certain diseases go overlooked is that they are often misidentified in the first place. According to Wharton, such is often the case with the distinction between early blight and brown leaf spot.

And the distinction is important in this case, because growers have been reporting failure of fungicides to control early blight. Although this could be explained by the existence of "resistant isolates" of the disease, another definite possibility is that in some cases the early blight is actually brown leaf spot, which is inherently more resistant to a number of traditional control products. Alternatively, fungicides such as difenoconazol plus mandipropamid (Revus Top) have been found to be much more effective in also controlling brown leaf spot.

There are a handful of ways to distinguish the two, but one of the most obvious signs is the characteristic banding pattern on the lesions of a plant infected with early blight.

Wharton notes that for brown leaf spot, the lesions consist of small dark brown spots with chlorotic margins, and the lesions may coalesce, creating large dead patches.

For management of both, Wharton suggests that growers avoid irrigation in cool, cloudy weather and time irrigation to allow plants time to dry before nightfall.

Protectant fungicides recommended for late blight control (such as maneb, mancozeb, chlorothalonil and triphenyl tin hydroxide) are also effective against brown leaf spot when applied at approximately 7- to 10-day intervals.

In addition to Revus Top, other fungicides that have shown efficacy against early blight include famoxodone plus cymoxanil (Tanos), pyrimethanil (Scala) and boscalid (Endura). Spraying should commence at the first sign of disease or immediately after bloom.

Wharton added that "early-season applications of fungicides before secondary inoculum is produced have minimal or no effect" on the spread of the disease. Growers can tackle this fungus with relatively few fungicide applications if the initial treatment is properly timed.

Once you are attuned to the precursors and circumstances that are favorable to any of these pathogens, identification and response should be a straightforward matter.

"You can extrapolate much of this to just about any disease out there," Nolte said. "Every kind of disease you're dealing with in potatoes has certain kinds of environmental conditions that are favorable for it to develop, and some that aren't. It's only when you have that perfect storm that some diseases show themselves and have an effect on the quality profile of your crop."

Note: This series is based on a seminar presented by Jeff Miller of Miller Research LLC (Rupert, Idaho), UI Extension Seed Specialist Phil Nolte and UI Potato Pathologist Phillip Wharton. In Part II of this series we will look at research findings concerning gray mold, black dot and white mold. Photos courtesy of U of I Extension, Miller Research and Oregon State University.

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